As an ex very long-time and proud Vonage customer, there is one reason I finally switched (sadly): Call quality. Vonage tried and tried but very frequently there were numerous different types of call quality issues. (many devices, different internet services, hours trouble shooting with Vonage engineering, etc.). I can say in the three years I've had Comcast voice, the one thing I have never had is the slightest call quality issue. True, they have yet to add an interesting feature, but it is truly a replacement for a traditional "home phone".
Specifically to the softphone question, I keep hoping for the same thing. One reason their call quality is so solid is because they control the most VoIP-sensitive parts of the network (and now the bulk of the rest of the path a call may take). If they gave you a soft phone, call quality is subject to whatever path you took all the way in to Comcast, AND the path it takes out to the other caller. When you are at home, they own the pipe all the way to your modem and therefore avoid the problems the plague the free-standing innovative VoIP providers.
I really loved Vonage but I wouldn't go back, at least not for a "home phone", since I work from home a lot and much of that time is on conference calls. Call quality issues are just not an option.
